With tributaries extending from the vast
savannas to its north and south, the Amazon River runs almost 4,000
miles (1 mile equals 1.6 kilometers) across northern South America from
the highland biomes in the foothills of the Andes Mountains to the
Atlantic Ocean. It carries twenty percent of all river water discharged
into Earth’s oceans—ten times the volume of the Mississippi River.
If the Amazon River Basin were draped over the continental United
States, it would cover more than three fourths of the country.

From December to May each year,
torrential rains and snow melt from the Andes increase the main river
channel’s depth 30-45 feet (9 to 14 meters), and water backs up in
tributaries and inundates forest several miles from the main channel.
In the central Amazon Basin alone, the flood waters can cover an area up
to 97,000 square miles. The river and the flooded forests then come
together as a giant, slow-moving swamp. Surrounding these waters are
over 2.7 million square miles (7 million square kilometers) of lush
forest exploding with life. In fact, perhaps as much as one half of all
life forms on the planet live in the Amazon River Basin.

The Amazon is more than a habitat, however; it is also a climate
regulator. Located near the equator, where the sun’s daily rays are most
intense, the uninterrupted expanse of lush vegetation absorbs incoming
radiation and keeps things cool. The forest also absorbs and stores
moisture. The Amazon forest canopy is so dense and so biologically
productive that scientists have also recognized the region as a key
component of the global carbon cycle. The continent-spanning tracts of
forest inhale tons of carbon dioxide during photosynthesis and exhale
oxygen. With respect to carbon, however, these forests aren’t all
take. Through deforestation, decomposition, respiration, and export of
organic and inorganic matter to the oceans, they also give.

And then there’s the rain. The Amazon
Rainforest engages in a perpetual, self-watering cycle by storing and
recycling at least one-half—some scientists think perhaps even
two-thirds—of the regional rainfall. This rain is more than just
recycled water—it is the ecosystem’s mechanism for venting
the tremendous amount of heat it collects and stores every day.
Earth’s ongoing attempt to redistribute the intense radiation and
heat it receives at the equator across the entire planet is the driving
force of climate. Much of that redistribution occurs within the
Earth’s oceans. But the atmosphere is also engaged in a continuous
process of spreading the heat around the globe, and tropical rainfall
drives the process.

The intense solar radiation reaching the Earth at the equator
provides the energy needed to evaporate huge volumes of water from both
the ocean and land surfaces. The water vapor released into the
atmosphere stores within it the heat energy that was required to turn
liquid water to water vapor. So what happens to all that latent heat?
It’s released back into the atmosphere when water vapor condenses
into clouds and rain. Given that more than 9 feet (2.7 meters) of rain
fall in portions of Amazonia each year, we are talking about a lot of
heat. In fact, 75 percent of the energy that drives atmospheric
circulation comes from the heat released during tropical rainfall.

The Amazon River runs almost 4,000 miles from the Andes Mountains in
the west to the Atlantic Ocean in the east. The river basin plays a key
role in heat, moisture, and carbon cycles both regionally and globally.
The region is also the most biologically diverse location on Earth,
supporting perhaps half of all species on the planet.

The dense vegetation of
the rainforest canopy
is a sink for atmospheric carbon dioxide.

Deforestation (top) and dissolved
organic material in the river outflow (bottom) are among the ways carbon
is lost. (Photographs courtesy Jeffrey Richey)

Because the dynamics of the Amazon River
Basin play a major role in environmental conditions that affect the
whole Earth, changes in climate and land use in the Amazon take on
global importance. During the past 15 years more than 190,000 square
miles (1 square mile is about 2.6 square kilometers) of forest have been
cleared in the Amazon Basin, and 7,700 square miles are now being
cleared each year. Hundreds, perhaps thousands, of future plants,
animals, mushrooms, and insects have already been lost. Deforestation
alters the ancient forests’ exchanges of water, carbon, and energy with
the atmosphere—cycles that even now, we only partially understand. What
are the impacts of land cover change in Amazonia both locally and
globally?

In 1993, the Brazilian science community, joined by an international
team of scientists, began to plan a continental-scale study to answer
that question. They established the Large-scale Biosphere-Atmosphere
Experiment in Amazonia (LBA) to study how Amazonia currently functions
as a regional entity within the larger Earth system, and how changes in
land use and climate will affect the biological, physical, and chemical
functioning of the region’s ecosystem.

Located on the equator,
Amazonia has a rainy season and a dry season, instead of the four seasons
of temperate latitudes. The rainy season starts in the south in December,
and gradually moves north through May. During this time the rains swell
the rivers of Amazonia, creating extensive wetlands. (Images by Robert Simmon,
based on data from the Global Precipitation Climatology Project)

The project, which is led by the Brazilian Ministry of Science and
Technology, began in 1998 and today
consists of 100 coordinated research groups involving about 600
scientists from South and North America, Europe, and Japan. With the
Amazon as their laboratory, the scientists are studying climate,
atmospheric chemistry, the carbon cycle, nutrient cycling, land surface
hydrology and water chemistry, land use and land cover, and the
interaction of humans with the landscape. NASA’s Terrestrial Ecology and
Land Use-Land Cover Change Programs participate in LBA through their
sponsorship of LBA projects called LBA-ECO (formerly LBA-Ecology).

As LBA is completing its fourth year, there are some exciting new
results, so check back here often as the Earth Observatory presents a
series of feature articles based on some of the project’s most
significant findings to date: the discovery of a large
source of carbon dioxide emissions within an ecosystem long thought to
be a carbon sink; the impact of development and progress on the rainforest’s flammability; and
the cloud and rainfall characteristics that have led some scientists to
dub the forests “the green ocean.”

LBA sites span the Amazon from the headwaters
in the Andes, along the river and its tributaries in the Amazon Basin, to the River’s
mouth in coastal Brazil. (Map courtesy LBA science team, adapted by Robert Simmon)